1. Field of the Invention
The present invention relates to an obstacle detection system for detecting an obstacle by utilizing a change in the static capacitance and a method thereof. More particularly, the present invention relates to an obstacle detection system and a detection method thereof that can provide an improved detection accuracy and an improved reliability by utilizing a phase locked loop.
2. Description of the Related Art
The doors of a vehicle are to be opened and closed directly by hand in the past. However, the number of vehicles having doors that can be opened and closed remotely by operating a button has been and is increasing.
In accordance with such a trend, there is an increasing demand for obstacle detecting systems that can interrupt the operation of closing a door or a window of a vehicle immediately when an obstacle is detected and drive the door or the window, whichever appropriate, in the opposite direction in order to prevent the obstacle, which may be a human hand or human body, from being pinched, if partly, in the door or the window, whichever appropriate.
Obstacle detection systems are roughly classified into the contact type and the non-contact type. Obstacle detection systems of the contact type detect a change in the electric load or the air pressure that arises when an obstacle contacts a door or a window and determine the presence or absence of an obstacle. On the other hand, obstacle detection systems of the non-contact type detect the presence or absence of an obstacle before a door or a window touches an obstacle. Obstacle detecting systems of the contact type that utilize a static capacitance is in the main stream as disclosed in Korean Patent unexamined Publication No. 10-2004-0041697.
FIG. 1 of the accompanying drawings schematically illustrates the configuration of a known obstacle detection system designed to detect a change in the static capacitance. The illustrated static capacitance detection system includes a static capacitance detection module 12 for detecting a static capacitance, a control module 18 for determining the presence or absence of an obstacle, utilizing the signal output from the static capacitance detection module 12 and a transmission line 20 for transmitting the output signal of the static capacitance detection module 12 to the control module 18.
The static capacitance detection module 12 by turn includes one or more sensor strips 14 arranged along the periphery of a door or a window and one or more static capacitance detection circuits 16, which or each of which is connected to an end of the sensor strip or each of the sensor strips 14 to detect the static capacitance of the sensor strip or each of the sensor strips 14, whichever appropriate.
A sensor strip 14 is formed by inserting a thin strip-shaped conductor into a highly flexible insulator typically made of rubber and the conductor operates as an electrode of the corresponding capacitor. Thus, the static capacitance of the sensor strip 14 varies depending on if there is an obstacle in the vicinity thereof or not.
While two sensor strips 14 are shown in FIG. 1, the number of sensor strips 14 is by no means limited to two and they may be arranged anywhere around a door or a window. While the sensor strips 14 are linear in FIG. 1, they may be bent or warped depending on the positions where they are arranged.
As schematically illustrated in FIG. 2 of the accompanying drawings, the static capacitance detection circuit 16 or each of the static capacitance detection circuits 16 is connected to a sensor strip 14 and includes an RF oscillator 42 for generating a high frequency signal of a variable RF oscillation frequency fRF, a local oscillator 40 for generating a signal of a local oscillation frequency fLO, a mixer 44 for mixing the output signal of the RF oscillator 42 and the output signal of the local oscillator 40 to produce a signal of a frequency equal to the sum of the above frequencies, or fLO+fRF, or the difference of the above frequencies, or fLO−fRL, a low pass filter 46 for passing only a low frequency signal out of the output signal of the mixer 44 and an amplifier 48 for amplifying the output of the low pass filter 46 among others.
The control module 18 includes a micro control unit (MCU) and, upon receiving the output signal of the static capacitance detection circuit 16, takes a role of determining if the signal is cut of an allowable range or not, comparing the output signal with a reference value. If the output signal of the static capacitance detection circuit 16 is determined to be out of the allowable range as a result of referring to the reference value, it transmits a control signal for automatically opening or closing a door or a window to a corresponding opening/closing module 30 in order to interrupt the motion of the door or the window, whichever appropriate, or drive it in the opposite direction.
The local oscillation frequency fLO of the local oscillator 40 and the RF oscillation frequency fRF of the RF oscillator 42 are determined at the time of designing the circuit. In other words, the output signal of the static capacitance detection circuit 16 shows a constant frequency in the normal condition where no obstacle is detected.
However, as a human hand or a human body approaches the door or the window, if partly, the static capacitance of the sensor strip 14 changes and hence the oscillation frequency of the RF oscillator 42 connected to the sensor strip 14 is changed.
As the oscillation frequency of the RF oscillator 41 is changed and the frequency of the output signal of the static capacitance detection circuit 16 goes out of the allowable frequency range, the control module 18 determines that there is an obstacle in the vicinity of the door or the window.
For example, if the RF oscillation frequency fRF of the RF oscillator 42 is 920 MHz and the local oscillation frequency fLO of the local oscillator 40 is 925 MHz, the frequency of the output signal of the static capacitance detection circuit 16 is held to 5 MHz in a state where there is no obstacle.
If the RF oscillation frequency fRF of the RF oscillator 42 is changed to 916 MHz, for instance, in this state, the frequency of the output signal of the static capacitance detection circuit 16 is changed to 9 MHz and hence the control module 18 recognizes that there is an obstacle in the vicinity of the door or the window from the change.
Obstacle detection systems of the above-described type are accompanied by a number of problems as listed below.
Firstly, while the oscillation frequency of the RF oscillator 42 and that of the local oscillator 40 are determined by way of a frequency tuning operation in the process of manufacturing the static capacitance detection circuit 16 but the accuracy of such a tuning operation depends on the experience of the operator and involves trial and error so that such an operation constitutes a significant limiting factor relative to productivity.
More specifically, a frequency tuning operation is performed by adjusting the inductance L and the capacitance C of the wiring to an oscillator, of which the inductance L is adjusted by changing the length of a copper foil (pattern) and the capacitance C is adjusted by changing the area of the copper foil. Thus, a frequency tuning operation is an operation for an operator of cutting a copper toil (pattern) to a predetermined length.
This will be described further by referring to FIG. 3 of the accompanying drawings that shows the wiring pattern of the printed circuit board (PCB) of a static capacitance detection circuit 16. The operation of tuning the local oscillation frequency fLO of the local oscillator 40 proceeds as the operator cuts the wire in the local oscillator tuning section 60 near the lower end of the PCB, checking the oscillation frequency fLO.
On the other hand, the operation of tuning the RF oscillation frequency fRF of the RF oscillator 42 proceeds as the operator adjusts the length of the wire in the RF oscillator tuning section 62, checking the oscillation frequency fRF.
Secondly, the reliability of a static capacitance detection circuit 12 can fall with time. More specifically, if the oscillation frequencies fRF and fLO are properly tuned in the circuit manufacturing process, they change gradually with time because the PCB and the sensor strip 14 are degraded with time and/or for some other reasons. Then, operation errors can take place to the static capacitance detection circuit 12.
If an operation error takes place for such reasons, it is highly difficult for the user to tune the oscillation frequencies and hence he or she will have to buy a new one.
Thirdly, there is a problem that there exist cyclic dead points in the operating area of a static capacitance detection circuit where any approaching obstacle cannot be detected.
According to the results of some experiments, dead points appear at regular intervals of about 14 cm when an RF oscillator of 900 MHz is employed and at regular intervals of about 28 cm when an RF oscillator of 450 MHz is employed. While the exact cause of appearance of such dead points is not known yet, it is presumed that the sensor strip 14 may operate like an antenna for the RF oscillator and hence the dead points may be related to λ/2 of the oscillation signal.
Therefore, the problem that an obstacle is not detected can be prevented to some extent by lowering the oscillation frequency to increase the intervals of dead points or by raising the oscillation frequency to minimize the intervals of dead points. For example, the intervals of dead points can be increased to about 280 cm by lowering the oscillation frequency to 40 MHz so that a smaller sensor strip can be employed. On the other hand, the intervals of dead points can be reduced to about 1.4 cm by raising the oscillation frequency to 9 GHz so that an obstacle having a width of 1.4 cm can be satisfactorily detected.
However, when the oscillation frequency is lowered, the oscillation frequency changes only to a small extent when an obstacle approaches the sensor strip so that the static capacitance detection circuit 12 shows a poor sensitivity. Then, it may sometime not properly detect an obstacle. When, on the other hand, the oscillation frequency is raised too much, the manufacturing cost will rise and design difficulties will become enormous among other problems.